Indirect evaporative cooling heat exchanger

ABSTRACT

A heat exchanger including a header having a plurality of header openings with conduits that may be made of plastic are inserted in the openings. The conduits are sealed to the header to prevent leakage between the header and the tubes to prevent water and air leakage between the wet, scavenger air stream flowing through the tubes and a dry air stream flowing around the tubes. A method of making the heat exchanger includes providing the openings inwardly curving perimeter walls that allow for easy insertion of the tubes. A self-leveling sealant may be used to seal the heat exchange conduits to the header using, for example, a paint roller and/or a paint sprayer. The heat exchange conduits can include a plurality of stubby fins formed on the inner wall surface of the conduit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the filing date benefit of U.S. Provisional Application No. 61/791,180, filed on Mar. 15, 2013, the entirety of which is incorporated herein by reference.

BACKGROUND

1. Field of Invention

This invention relates to indirect evaporative cooling technology, and particularly to heat exchangers useful in indirect evaporative cooling devices used for conditioning air. U.S. Pat. No. 7,128,138 B2, filed May 26, 2004, and U.S. Pat. No. 7,716,829, filed May 18, 2010, relate to similar technical fields and are hereby incorporated by reference in their entireties.

2. Description of Related Art

Evaporative cooling involves lowering the temperature of a liquid by utilizing the latent heat of vaporization of a portion of the liquid. The term “Indirect Evaporative Cooling” was coined by personnel at Des Champs Laboratories in 1974, when they decided to enhance summer-time air-to-air energy recovery, from building exhaust air, by utilizing the wet bulb temperature of the exhaust air instead of the higher dry bulb temperature. At the time, it was common practice during summer months to transfer energy from the cooler exhaust air to the warm, outdoor, make-up air by using an air-to-air heat exchanger. The driving force that causes the transfer of energy within the heat exchanger, in the aforementioned process, is the sensible temperature difference between the two air streams. During summer months, the outdoor air that is delivered to a space, and the recirculated internal air, are usually air-conditioned. As a result, the air within the space has a lower wet bulb temperature than the outdoor air or the inside dry bulb temperature.

By spraying water on the surface of the exhaust side of the air-to-air heat exchanger during the cooling season, the exhaust air flow, at a low wet bulb temperature, evaporates water from that exhaust side surface and thereby attempts to drive the water/exhaust-side surface temperature lower, approaching the exhaust air wet-bulb temperature at the limit. The supply air, flowing on the other side of the membrane that separates the two air streams, comes in contact with a surface (the opposite side of the membrane from the exhaust side) that is much cooler, and consequently more energy is transferred between air streams and thus a greater energy saving occurs. The reason the surface is cooler than it would otherwise be is because of the evaporative cooling that takes place at the exhaust air/water layer interface, which in turn manifests itself as a cooler membrane temperature than would exist if the exhaust air were simply left dry with no water spray. The temperature drop across the membrane, from the exhaust-side surface to the supply-side surface, is very small, i.e., on the order of a fraction of a degree, while the typical temperature difference between the two bulk air streams is on the order of 10 to 40° F.

Early indirect evaporative cooling (IEC) units were simply a modification of standard air-to-air heat exchangers that were used to extract energy (or lack of energy) from the exhaust air and transfer it to fresh, incoming make-up air, thus reducing the energy that would otherwise be required to condition the outdoor air prior to delivering it to the occupied space. Consequently, the heat transfer devices used in the early IEC units were designed to transfer energy in a dry environment. In contrast, more recent IEC units are subjected to a wet environment. Such wet environments are known to contain a wide range of contaminants and are often corrosive to IEC components. As a result of the hostile environment that such IEC heat exchangers witnessed, they were maintenance prone and short lived. Consequently, IECs, after getting off to an admirable start in the late 1970s and early 1980s, languished in the 1990s and, likewise into the new century so far, even though IECs have the potential for tremendous energy savings and reduction in peak summer electrical demand.

More recently, known heat exchangers have been constructed at least partly of plastic materials, including, for example, corrosion resistant polymers, to deal with corrosion common in wet environments. Although these plastic materials have prolonged the lifespan of IEC units, some plastics have low thermal conductivity, resulting in lower heat transfer across a surface.

Additionally, known heat exchangers have designs that require lengthy assembly periods. For example, in known systems, assembly of heat exchanger tubes to a plate or manifold requires an individual to seal around the perimeter of each tube by hand in an attempt to prevent leaks. This method of assembly often requires 10-20 hours to implement. Furthermore, extensive quality assurance is also necessary due to the possibility of leaks.

SUMMARY

The present invention is directed to improvements in indirect evaporative cooling technology. Exemplary improvements include a novel heat exchanger useful in indirect evaporative cooling devices used for conditioning air. In one exemplary embodiment of the invention, an air-to-air heat exchanger designed specifically for use in hostile environments associated with the application of IECs in wet environments is provided.

Because the pH level of water varies from acidic to alkaline depending upon the geographic location of the unit, the present invention uses materials that function properly over the varying pH levels of water, such as, for example, plastic as a suitable material with which to construct the IEC heat exchanger.

Additionally, because plastic polymers have low thermal conductivity, in some embodiments, the thermal conductivity of the plastic is enhanced by adding thermally conductive particles such as carbon nano-particles. For example, heat-conductive additives that may be added to a plastic polymer material include graphite carbon fibers, ceramics such as aluminum nitride and boron nitride, and other suitable additives. This can increase the thermal conductivity of the plastic polymer by 5 to 50 times the thermal conductivity of conventional plastics. In some embodiments, thermal transfer properties may be enhanced by decreasing the thickness of the material. For plastic polymers, the thickness of the material may be determined by the extrusion process and the strength of the polymer material. The heat exchanger tubes may be made of a plastic polymer such as polyvinylchloride. In some embodiments, the entire heat exchanger is made of a plastic polymer. Furthermore, because water can be very hard, i.e., have a high mineral concentration, IEC heat exchangers according to this invention are designed to be relatively unaffected by water hardness and the possible resulting material build-up within the heat exchanger.

Various embodiments of the systems and methods according to this invention provide IEC heat exchangers that are relatively economical to manufacture and relatively quick to assemble.

Various embodiments of the systems and methods according to this invention provide IEC heat exchangers that are relatively no more maintenance prone than a common air-conditioner.

Various embodiments of the systems and methods according to the invention separately provide IEC heat exchangers that serve simultaneously as integral cooling towers and air-to-air heat exchangers.

Various embodiments of the systems and methods of manufacture according to the invention separately provide means of containing cooling water in areas that the water is intended to be so as to perform the necessary thermodynamic functions of an IEC heat exchanger.

Various embodiments of the systems and methods according to the invention separately provide IEC heat exchangers which tend not to degrade because of high or low pH water in contact with a surface of the heat exchanger.

Various embodiments of the systems and methods according to the invention separately provide IEC heat exchangers that have a wet side surface that is wettable, can be kept free of mineral deposits even though hard water is intermittently sprayed on a surface of the heat exchanger, and can be manufactured at a cost that allows an IEC containing the heat exchanger to compete ton for ton of air conditioning with standard mechanical air conditioning.

Various embodiments of the systems and methods according to the invention separately provide IEC heat exchangers that have a unique heat exchange tube design. For example, in some embodiments, the heat exchanger tube may have a plurality of short and stubby fins formed on an inner wall surface. In some embodiments, the external width of the tube may be between 5 to 12 times a height of the plurality of stubby fins, preferably between 7 to 10 times, and more preferably between 8 to 9 times. In some embodiments, a height of each of the plurality of fins is greater than a length of the base of a fin.

Various embodiments of the systems and methods according to the invention separately provide IEC heat exchanges having a unique connection and/or seal between heat exchange tubes and the side walls of a heat exchanger. For example, an important aspect of heat exchanger assembly is the installation and adhesion of a plurality of heat exchanger tubes to two opposing plates of the heat exchanger. Adequate adhesion is important for protecting against unwanted air and water leakage between the dry supply air stream and the wetted, humid exhaust air stream. For example, in some embodiments, a heat exchanger may include: a first side plate and a second side plate, the first side plate including a first outside surface and a first inside surface, and the second side plate including a second outside surface and a second inside surface; at least a first opening in the first side plate, and at least a second opening in the second side plate corresponding to the first opening, configured to allow at least one heat exchanger tube to pass though; and the first outside surface defining a first opening perimeter wall around the first opening and the second outside surface defining a second opening perimeter wall around the second opening. The first opening perimeter wall may curve inwardly toward the first inside surface of the first side plate, and the second opening perimeter wall may curve inwardly toward the second inside surface of the second side plate. The first opening perimeter wall around the first opening that curves inwardly toward the first inside surface, and similarly, the second opening perimeter wall around the second opening that curves inwardly toward the second inside surface, may include a first portion that curves toward the corresponding opening, a second portion that curves away from the corresponding opening and a third portion that connects the first portion and second portion, the first portion being closer to the first outside surface of the first side plate, and similarly, the second outer surface of the second side plate, than the second and third portions. A heat exchanger tube that is inserted into the first opening, and extends through to the second opening, may allow the tube to contact the third portions of the opening perimeter wall of first opening and the opening perimeter wall of the second opening and allow for a slight interference fit. Once the tube is flush with the first side plate and/or second side plate, sealant may be applied by a simple, time-efficient process, e.g., by rolling a common paint roller across the surface of the first side plate and/or second side plate.

Various embodiments of the systems and methods according to the invention provide IEC heat exchange methods of manufacture that achieve relatively low cost assembly of heat exchanger components and easy repeatability of manufacture of such components by relatively unskilled labor.

Various embodiments of the systems and methods according to the invention provide IEC heat exchangers having an improved interface between a heat exchange tube and the side walls of a heat exchanger.

Various embodiments of the system and methods according to the invention provide IEC heat exchangers having an improved method of removing collected minerals and solids that, for example, may be washed out from the heat exchanger tubes. For example, in some embodiments, a sump may be placed below the heat exchanger to collect minerals and corrosives that are byproducts of evaporation and that have washed out of the heat exchanger tubes. The sump may be made of a plastic polymer to protect it from the pH and corrosive minerals in water. The sump may be made from the same plastic polymer that the heat exchanger tubes are made of. The sump may be formed into a single piece to prevent water leakage. The sump may be formed from a plurality of different pieces. The sump may be chemically bonded to at least one wall of the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an indirect/direct evaporative cooling unit, including an indirect evaporative cooler heat exchanger;

FIG. 2 shows a perspective view of a heat exchanger;

FIG. 3 is a schematic drawing of an indirect evaporative air conditioning process using indirect evaporative cooling;

FIG. 4 shows a partial view of side plate openings and a plastic tube partially inserted in an opening;

FIGS. 5A and 5B show a cross-sectional view of a plastic tube inserted into a heat exchanger;

FIGS. 6A and 6B are schematic views of a cross-section of a plastic tube;

FIG. 7 is a schematic view of a cross-section of a plastic tube showing the dimensions of the internal fins;

FIG. 8 is a graph showing the heat exchange performance of the fins on the internal surface of the tubes for different heights of fins; and

FIG. 9 is a flow chart showing an exemplary method of assembly of an IEC heat exchanger.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a perspective view of an indirect/direct evaporative cooling unit 1 according to an exemplary embodiment of the invention. As shown in FIG. 1, an indirect/direct evaporative cooling unit 1 includes a heat exchanger 10, a base portion 2, a frame 3, an air extraction fan 4 mounted in a top wall of the frame 3, and a sump 5 disposed below the heat exchanger 10. The frame 3 surrounds the heat exchanger 10, and the air extraction fan 4 removes high energy, unconditioned exhaust air, which may be scavenger ambient-air, that has been routed through the heat exchanger 10, as will be discussed later. The base portion 2 and frame 3 may be made of, for example, metal, stainless steel, coated aluminum, plastic, plastic polymer, or any other suitable material.

A support frame 6 is mounted to the base portion 2 within the unit 1 to provide a mounting point for the indirect heat exchanger 10 and a direct cooling stage 7 (FIG. 3). A water distribution manifold 8 (FIG. 3) is disposed over the indirect heat exchanger 10 to deliver water thereto.

The sump 5 is disposed within the support frame 6 and below the heat exchanger 10, and functions as a drain pan for collecting minerals and corrosives that are washed out from the heat exchanger 10. The sump 5 may be shaped as one long tray, and the tray may have multiple levels. For example, the sump 5 may have a plurality of top trays 51 and at least one bottom tray 52. The top trays and/or the bottom tray may have a plurality of ridges. The top trays 51 may be angled so that minerals and corrosives that are collected in the top trays preferentially flow to and collect in the bottom tray 52. The sump 5 may be removable from the unit 1. The sump 5 may be integrally foamed on the unit 1. The sump 5 may comprise, for example, metal, stainless steel, coated aluminum, plastic, plastic polymer, or any other suitable material. The sump 5 may be formed of a single piece of material. The sump 5 may be welded from several pieces of material.

FIG. 2 shows a perspective view of an indirect heat exchanger 10, according to an exemplary embodiment of the invention. The heat exchanger 10 includes a first side plate 20 that includes a plurality of first openings 21, and a second side plate 30 that includes a plurality of second openings 22 (not shown). The unit 1 also includes water distribution manifold 8, which includes a pump 11 that pumps water to the top of the heat exchanger 10 through a series of water pipes 12. The water that is pumped to the top of the heat exchanger 10 is sprayed onto the outside of heat exchanger tubes 40 so as to evaporatively cool the tubes 40, as discussed below. In some embodiments, the tubes 40 are disposed to be substantially horizontal in relation to the ground. By having horizontal tubes 40, for example, the tube length can be selected to maintain a low pressure drop so that a low cost and compact propeller-type fan can be used as the air extraction fan 4. In addition, horizontal tubes 40 may allow for better structural support for the tubes 40, reduction of overall height of the heat exchanger 10, better distribution of cooling water from the water distribution manifold 8 to fall on the tube surfaces, and less resistance to exhaust air 42 flowing upward with the water flowing downward. In some embodiments, the tubes 40 are disposed to be substantially vertical in relation to the ground.

In an exemplary embodiment of the invention, the first side plate 20 and the second side plate 30 may comprise, for example, metal, stainless steel, coated aluminum, plastic, plastic polymer, or any other suitable material. In some embodiments, the first side plate 20 and/or the second side plate 30 may be made from a ⅛-inch thick aluminum sheet.

FIG. 3 is a schematic of the indirect evaporative air conditioning process using indirect evaporative cooling, according to an exemplary embodiment of the invention. As shown in FIG. 3, hot supply air 41 (e.g., recirculated internal air), is blown through the inside of the heat exchanger tubes 40. At the same time, water from the water distribution manifold 8 is pumped by pump 11 through the pipes 12, and sprayed onto the outside of the tubes 40. In addition, at the same time, the air extraction fan 4 is turned on to draw exhaust air 42 into the heat exchanger 10 and over the outside of the tubes 40, due to the pressure drop within the heat exchanger 10. In some embodiments, scavenger ambient-air is drawn into the heat exchanger 10 and over the outside of the tubes 40.

As water flows downwardly across the outside of the tubes 40, the exhaust air 42 flows upwardly across the outside of the tubes 40. Thus, the exhaust air 42 serves as air that evaporatively cools the outside of the tubes 40. The exhaust air 42 exits the heat exchanger 10 through the air extraction fan 4 as moistened exhaust air 42. In some embodiments, the exhaust air 42 exits the unit 1 through an exhaust grate (not shown).

The hot supply air 41 is blown through the inside of the heat exchanger tubes 40, for example, by use of a fan or other suitable means. The hot supply air 41 does not come into direct contact with the water and air outside the tubes 40. As the hot supply air 41 flows through the inside of the heat exchanger tubes 40, heat is extracted by the tubes 40 that have been evaporatively cooled, and cool supply air 43 exits the tubes 40. Therefore, the hot supply air 41 is cooled without having water added, and the hot supply air 41 exits the exchanger 10 as dry conditioned air.

In operation, the cool supply air 43 then may flow into a direct cooling stage 7 comprised of a suitable high quality evaporative medium, such as “CELDEC”, available from Munters Corporation.

FIG. 4 is a partial view of a heat exchanger surface showing (i) empty side plate openings, (ii) side plate openings with plastic heat exchange tubes; and (iii) one plastic heat exchange tube partially inserted in an opening. As shown in the exemplary embodiment of FIG. 4, the outside surface of the first side plate 20 includes a plurality of first openings 21. The first openings 21 in the first side plate 20 include an opening perimeter wall 60 around the first openings 21 that curves inwardly toward the inside surface of the first side plate 20, as discussed below.

In an exemplary embodiment of the invention, the tubes 40 may be made of a plastic and formed by known extrusion processes. Such plastics include, for example, corrosion resistant polymers having a fire and smoke retardant rating that meets or exceeds UL94 V-O or V-1 rating.

FIGS. 5A and 5B show a cross-sectional view of a plastic tube inserted into a heat exchanger. Although only the first side plate 20 will be discussed in detail below, it can be understood that the second side plate 30 has the same construction. As shown in the exemplary embodiment of FIGS. 5A and 5B, the opening perimeter wall 60 of the first opening 21 of the first side plate 20 curves inwardly toward the inside surface of the first side plate 20, and the opening perimeter wall 60 of the second opening 31 of the second side plate 30 curves inwardly toward the inside surface of the second side plate 30. Preferably, the second opening 31 is positioned to correspond to the first opening 21, such that a straight tube can be positioned in both the first and second openings as shown in FIG. 5A.

As shown in FIGS. 5A and 5B, the portion of the perimeter wall 60 that curves inwardly includes a first portion 61 that curves toward the first opening 21, a second portion 62 that curves away from the first opening 21, and a third portion 63 that connects the first portion 61 and second portion 62. The first portion 61 of the perimeter wall 60 is closer to the outside surface of, for example, the first side plate 20, than the second portion 62. The third portion 63 provides an interface between the first side plate 20 and the tubes 40 inserted into the first opening 21.

In the exemplary embodiment of the invention, the curved perimeter wall 60 aids in the insertion of the tube 40 into the first opening 21, allowing the tube 40 to contact the third portion 63 of the perimeter wall 60 of first opening 21, to allow for a slight interference fit between the tube 40 and the first opening 21. Additionally, for example, by providing a first side plate 20 with openings 21 having such a perimeter wall 60, a more rigid plate construction is achieved.

In an exemplary embodiment of the invention, the perimeter wall 60 serves to allow for an approximately flush fit between a first end of the tubes 40 and the first side plate when the tubes 40 are installed in the first side plate 20, and/or an approximately flush fit between a second end of the tubes 40 and the second side plate when the tubes 40 are installed in the second side plate 30. For example, such a configuration enables a sealant 70 to be easily applied to the outer surface of the first side plate 20 and/or the second side plate 30 after the tubes 40 are installed.

As shown in FIGS. 5A and 5B, the tubes 40 are inserted into the first opening 21 of the first side plate 20 and the second opening 31 of the second side plate 30. The tubes 40 may be installed by press-fitting, or any other suitable method.

In an exemplary embodiment of the invention, the tubes 40 may be made of a plastic, as noted above. In some embodiments, the tubes 40 may be made of polymeric plastic material, e.g., polyvinylchloride. In some embodiments, a plurality of thermally conductive particles may be disposed in the polymeric plastic material. Such an embodiment may result in tubes 40 with higher thermal conductivity than conventional plastics. For example, the thermal conductivity of the tubes 40 may increase to 5 to 50 times the thermal conductivity of conventional plastics as a result of the addition of the thermally conductive particles. This would allow for greater heat transfer between the hot supply air 41 flowing through the inside of the tubes 40 and the exhaust air 42 flowing over the wet outside surface of the tubes 40.

As shown in FIG. 5B, upon installation of the tubes 40 in the heat exchanger 10, a sealant 70 is applied over the outer surface of the first side plate 20 and/or the outer surface of the second side plate 30. For example, on the first side plate 20 side, the sealant 70 fills any gaps between a first end of the tube 40 and the first portion 60 of the perimeter wall 60 of the first opening 21, and may also fill any gaps between the tube 40 and at least a part of the third portion 63. The sealant 70 may be a liquid adhesive, such as liquid Vulkem®, which is a self-leveling sealant, or other suitable adhesive.

Sealing the outer surface of, for example, the first side plate 20, serves to prevent water and air leakage between the wet, exhaust air 42 flowing around the outside of the tubes 40 and also serves to hold the tubes 40 flush with the outer surface of the first side plate 20.

Because the tubes 40 are held approximately flush with the outer surface of the first side plate 20, the sealant 70 may be applied by rolling the sealant 70 onto the outer surface of the first side plate 20, such as with a common paint roller. By applying the sealant 70 using such a method, the time needed to seal the tubes 40 to the first side plate 20 is significantly reduced. For example, applying the sealant 70 with a roller may take approximately 5-10 minutes, or less, depending on the size of the first side plate 20. In contrast, known designs of heat exchangers require approximately 10-20 hours to seal heat exchanger tubes to a plate or manifold.

In another example, using a self-leveling single component liquid urethane applied with a six inch wide roller to seal around each tube of a side plate having approximately 44 tubes requires about thirty seconds. In contrast, applying a known “gun grade” sealant from a caulking gun to seal around each tube of a header plate having approximately 44 tubes requires approximately four minutes. The time differential between the two techniques increases as the size of the side plate and the number of tubes increases.

Although these examples describe applying the sealant with a roller, other methods of applying the sealant are within the scope of this invention. For example, the sealant 70 may be sprayed on to the outer surface of the first side plate 20 and the tubes 40, thereby significantly reducing the time required to seal heat exchangers over known methods.

The above-discussed configuration has numerous advantages. For example, the inward curvature of the perimeter wall 60 around the first opening 21 on the first side plate 20, and similarly, the inward curvature of the perimeter wall 60 on the second opening 31 on the second side plate 30, allows for the tubes 40 to be easily inserted into and pass through the first opening 21 (or, alternatively, the second opening 31), and also to be easily recovered and slid through the corresponding second opening 31 (or, alternatively, the corresponding first opening 21). In addition, the inward curvature of the perimeter walls 60 around the first and second openings 21 and 31 allow for a better interference fit between the circumference of perimeter walls 60 and the tubes 40 by allowing the tubes 40 to fit snuggly between opposing portions (around the circumference) of the perimeter walls 60. Accordingly, this configuration allows for easy and time-efficient installation of the tubes into the heat exchanger 10, thus reducing the costs and time necessary for manufacturing the IEC system. Further, for example, the inward curvature structure of the perimeter walls around the openings allows for greater surface area, and thus greater contact area, for adherence of the sealant applied to the perimeter walls 60 of the first and second openings 21 and 31. This allows for better adhesion between the first side plate 20 and the end of the tube 40 disposed in the first opening 21, and similarly, between the second side plate 30 and the other corresponding end of the tube 40 disposed in the corresponding second opening 31. Thus, because the sealant 70 can be simply rolled across the surface of a side plate (with, for example, a common paint roller, as discussed above), this configuration allows for maximum adhesion while reducing the time necessary for manufacturing the IEC system.

FIGS. 6A and 6B are schematic views of a cross-section of a plastic tube, according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, the tubes 40 are essentially ovoid in shape and may have an external chord length between 1 to 5 inches, and preferably of about 3 inches, and an external width between 0.1 and 0.5 inches, and preferably of about 0.375 inches. The tubes 40 may also have a wall thickness of between 0.010 inches and 0.030 inches, and preferably of about 0.020 inches. A web 80 may be formed at substantially the center of the chord length. The web 80 is formed transverse to the narrow, elongated portions of the tube 40 and connects the sides of the tube 40 at substantially the center of the chord length. In the exemplary embodiment, the tubes 40 may range in length from about 24 inches to about 96 inches, for example, with the most common length being about 48 inches in length. In such an exemplary embodiment, the exchanger would have approximately 144-1000 tubes disposed therein. Although this exemplary embodiment includes the description discussed above, tubes 40 having other dimensions are contemplated.

In an exemplary embodiment of the invention, the web 80 aids in maintaining the dimensions of the tube 40 during handling and assembly of the heat exchanger 10. For example, the web 80 aids in maintaining the dimension of the width of the tube 40 as the tube 40 is inserted into the header 30. If the web 80 were not in place, the tube 40 would tend to draw up on its center and result in a tube width of less than, for example, the desired 0.375 inches of the exemplary embodiment, thus causing problems with sealing the tube 40 to the first side plate 20 and/or the second side plate 30. The result of not completely sealing the tube 40 to the first side plate 20 and/or the second side plate 30 is unwanted air and water leakage between the dry supply air stream 41 and the wetted, humid exhaust/scavenger air stream 42.

In an exemplary embodiment of the invention, the tubes 40 may include a plurality of short and stubby internal fins 90 formed on an inner wall surface of the tubes 40. The internal fins 90 provide a greater surface area from which heat may be extracted from the hot supply air 41 flowing through the tubes 40, thus increasing the cooling efficiency of the tubes 40. The internal fins 90 also allow the tubes 40 to have greater structural rigidity, thereby preventing ballooning or collapsing of the tubes 40, for example, as a result of fan pressure when the air extraction fan 4 (FIG. 1) helps channel the air flow over the outside of the tubes 40 and out of the unit 1.

In an exemplary embodiment of the invention, the tubes 40 may include a plurality of external fins 100 formed on an outer wall surface of the tubes 40. The external fins 100 aid in wetting the outer surface of the tubes 40 and providing a greater surface area from which water may evaporate to aid in increasing cooling efficiency.

FIG. 7 is a schematic view of a cross-section of a plastic tube showing the dimensions of the short and stubby internal fins, according to an exemplary embodiment of the invention. According to this embodiment, the addition of internal fins increases the surface of the internal surface of the tube, which allows for greater thermal transfer of the heat of the air passing internally through the tube. Generally, plastic materials are used for IEC heat exchangers because they are relatively unaffected by water hardness and the resulting material build-up within the heat exchanger. However, as discussed, above, some plastic materials have low thermal conductivity. As a result, during the manufacturing of heat exchangers, it was discovered that internal fins that are longer in length resulted in lower heat transfer, despite increased surface area, due to the low thermal conductivity of the plastic materials. On the other hand, short, stubby internal fins had numerous advantages. For example, short, stubby internal fins increased surface area while maintaining good thermal transfer properties of the plastic materials due to the short length of the fins. Thus, the short, stubby internal fins, along with the polymeric plastic material of the fins greatly improve the structure of an IEC heat exchanger because these features result in greater surface area within the tubes for improved heat transfer, while maintaining good heat transfer characteristics of the tubes.

As shown in FIG. 7, height L of the internal fins 90 is greater than the length t of the base (also referred to herein as the width dimension of the fins). For example, the ratio of the height L to the length t may be 3 to 1, and more preferably 2 to 1, and most preferably 3 to 2. The height L of the internal fins 90 may be between 0.02 inches and 0.1 inches, and more preferably between 0.04 inches and 0.08 inches, and most preferably about 0.06 inches. In relation to the external width of the tube (of about 0.375 inches, as discussed above), the external fins 90 are preferentially kept short in comparison to the external width of the tube. For example, the external width is about 5 to 12 times the height L of the internal fins 90, and more preferably about 7 to 10 times the height L, and most preferably about 8 to 9 times the height L. In addition, the length t of the base of the internal fins 90 may be between 0.01 inches to 0.1 inches, and more preferably between 0.03 inches to 0.07 inches, and most preferably about 0.04 inches.

As shown in FIG. 7, a space 91 is disposed between each two adjacent internal fins 90 so that the fins 90 are circumferentially spaced apart from each other, i.e., spaced apart on the perimeter direction of the inner wall surface. The space 90 may have a length b that is less than the length t of the base of the internal fins 90. The space 90 may have a length b that is more than the length t of the base of the internal fins 90. In some embodiments, the length b of the space 90 may be between 0.01 and 0.04 inches, and more preferably between 0.02 inches and 0.03 inches. In some embodiments, the ratio of the length b of the space 90 to the length t of the base of the internal fins 90 is about 1 to 3, and more preferably about 1 to 2.

As discussed above, the addition of internal fins 90 on the inner wall side of the tubes 40 provide a greater surface area for better heat extraction and lower thermal resistance. For example, the addition of internal fins 90 can increase the inside surface area of the fin by between 100% to 300%, and more preferably by about 250%. However, because the tubes 40 are made of a plastic material, and because plastic has low thermal conductivity, increasing the height L of the internal fins 90 beyond a predetermined length results in an increased pressure drop and less overall heat extraction. Thus, as shown in FIG. 8, as the height of the internal fins 90 increases, the performance (i.e., efficiency of heat exchange) increases until a height of around 0.06 inches is reach. At this point, the performance starts to drop with additional increases in the height of the internal fins 90. Thus, in the exemplary embodiment, it is preferential to have short, stubby internal fins 90 on the inside surface of the tubes 40 for maximum efficiency of heat exchange for the most cost-efficient amount of materials.

In an exemplary embodiment of the invention, the walls of the tube 40 are designed with a strength that allows for a determined amount of transverse wall movement, or flex. For example, a determined amount of transverse wall movement, i.e., on the order of 0.025 inches, occurs in the tube wall when the pressure in the tube 40 is raised to 0.5-inches of water column pressure. As a result of such determined transverse movement, any solid deposits, such as mineral deposits or contaminant build-up on the inner surface of the wall, are separated from the wall surface when the pressure changes sufficiently to cause wall flex. For example, a sufficient pressure change may result when a fan (not shown) that blows air through the heat exchanger 10 is turned on or off. The deposits drop into a water sump 5 (FIG. 1) disposed below the base 2 of the heat exchanger 10 and are flushed from the system on a regular basis.

To further enhance the cooling effectiveness of the cooling unit 1, in some embodiments, baffle members (not shown) may be disposed in the space between adjacent tubes 40 so as to increase the surface area within the matrix of tubes 40 from which water can be evaporated. For example, the baffle members may comprise corrugated sheets, and the sheets may be made of a polymeric plastic material, e.g., polyvinylchloride. In some embodiments, to further enhance the cooling effectiveness of the cooling unit, an evaporative media (not shown) may be disposed at the exhaust air intake of the heat exchanger 10 so as to precool the exhaust air 42 prior to entering the heat exchanger 10. For example, evaporative media such as Munters Celdek pads can be used. In some embodiments, the evaporative media pads may be disposed below the base 2 of the heat exchanger 10, and may be disposed above a sump 5.

FIG. 9 is a flow chart showing an exemplary method of assembly of an IEC heat exchanger according to the invention. The method of manufacturing a heat exchanger begins in step S1000 and proceeds to step S1010 where heat exchanger tubes made, for example, of a suitable plastic material, are formed. As noted above, in one exemplary embodiment, the tubes have an ovoid shape. Then, in step S1020, a heat exchanger, with at least a first side plate and a second side plate, is formed which has relatively flat surfaces on the first side plate and the second side plate, the first side plate containing first openings and the second side plate containing second openings. Next, in step S1030, the first openings are provided with first perimeters having curved portions that curve toward the inside of the first side plate, and the second openings are provided with second perimeters having curved portions that curve toward the inside of the second side plate, to accommodate a rigid heat exchange tube snugly. Next, in step S1040, one or more fins are provided on the inside surface of the heat exchanger tubes. Then, in step S1050, a heat exchange tube is interference fit into each first opening and corresponding second opening. Next, in step S1060, the edge of each heat exchange tube is made flush with the exterior surface of the heat exchanger, e.g., the exterior surface of the first side plate and/or the exterior surface of the second side plate. Then, in step S1070, a sealant is applied to the exterior surface of the first side plate and/or the second side plate with inserted heat exchange tubes. Then, the process ends in step S1080. As noted above, a sealant may be applied using a paint roller and/or a paint sprayer, to reduce the manufacturing time of the IEC heat exchanger.

While the invention has been described in conjunction with exemplary embodiments, these embodiments should be viewed as illustrative, not limiting. Various modifications, substitutes, or the like are possible within the spirit and scope of the invention. For example, the invention may be used with or without direct evaporative coolers. 

What is claimed is:
 1. A heat exchanger for indirect evaporative cooling of a gas stream, comprising: at least one elongate heat exchange conduit, which accommodates fluid flow in an interior of the conduit, the conduit being made of a polymeric material and including an inner wall surface and an outer wall surface, the inner wall surface including a plurality of stubby fins arranged circumferentially along a perimeter of the inner wall surface and protruding into the interior of the conduit, the stubby fins having a maximum height dimension to maximum width dimension ratio in the range of from 3:1 to 1:1.
 2. The heat exchanger of claim 1, wherein the heat exchanger is configured so that gas flows through the interior of the heat exchange conduit, and so that liquid flows outside of the heat exchange conduit and contacts the outer wall surface.
 3. The heat exchanger of claim 1, wherein each of the stubby fins has a maximum height dimension to maximum width dimension ratio in the range of from 2:1 to 1:1.
 4. The heat exchanger of claim 1, wherein each of the stubby fins has a maximum height dimension to maximum width dimension ratio of about 3:2.
 5. The heat exchanger of claim 1, wherein each of the stubby fins has a maximum width dimension at a base, and the length of the base is from about 0.01 inches to about 0.3 inches.
 6. The heat exchanger of claim 5, wherein the length of the base is from about 0.03 inches to about 0.1 inches.
 7. The heat exchanger of claim 1, wherein the maximum height dimension of each of the plurality of fins is from about 0.02 inches to about 0.4 inches.
 8. The heat exchanger of claim 1, wherein the maximum height dimension of each of the plurality of fins is from about 0.04 inches to about 0.2 inches.
 9. The heat exchanger of claim 1, wherein each of the plurality of fins are circumferentially spaced apart from each other so that adjacent fins on the inner wall surface are separated by a distance b.
 10. The heat exchanger of claim 9, wherein the distance b is less than the maximum width dimension of each of the plurality of fins.
 11. The heat exchanger of claim 9, wherein the distance b is from about 0.01 inches to about 0.2 inches.
 12. The heat exchanger of claim 11, wherein a ratio of the maximum width dimension to the distance between adjacent fins b is in the range of from about 5:1 to 3:1.
 13. The heat exchanger of claim 1, wherein the at least one heat exchange conduit includes a plurality of outside fins formed on the outer wall surface.
 14. The heat exchanger of claim 1, wherein the polymeric material comprises polyvinylchloride.
 15. The heat exchanger of claim 1, wherein the at least one heat exchange conduit includes comprises a plurality of thermally conductive particles disposed in the polymeric plastic material.
 16. The heat exchanger of claim 15, wherein the plurality of thermally conductive particles comprises carbon nano-particles.
 17. The heat exchanger of claim 1, wherein the heat exchanger is configured such that air flows through the interior of the at least one heat exchange conduit, and air or water flows outside of the heat exchange conduit and contacts the outer wall surface.
 18. The heat exchanger of claim 1, further comprising a plurality of elongate heat exchange conduits that are spaced apart from each other in the heat exchanger.
 19. The heat exchanger of claim 18, further comprising at least one baffle member disposed between adjacent heat exchange conduits in the heat exchanger.
 20. The heat exchanger of claim 19, wherein the at least one baffle member comprises a polymeric material.
 21. The heat exchanger of claim 1, wherein the at least one heat exchange conduit is disposed substantially horizontally in the heat exchanger, and a thermally-formed sump is disposed below the at least heat exchange conduit to collect particles washed out of the heat exchanger.
 22. A heat exchanger for indirect evaporative cooling of a gas stream, comprising: at least one elongate heat exchange conduit; a first plate with a first outside surface and a first inside surface, the first plate having at least one first opening that defines a first opening perimeter wall in the first plate; and a second plate having a second outside surface and a second inside surface, the second side plate having at least one second opening that defines a second opening perimeter wall in the second plate, the second inside surface of the second plate opposing the first inside surface of the first plate, wherein: one end portion of the elongate heat exchange conduit is fitted into the first opening and in contact with the first opening perimeter wall, and another end portion of the elongate heat exchange conduit is fitted into the second opening and in contact with the second opening perimeter wall, the first opening perimeter wall curves inwardly from the first outside surface of the first plate toward the second inside surface of the second plate, and the second opening perimeter wall curves inwardly from the second outside surface of the second plate toward the first inside surface of the first plate.
 23. The heat exchanger of claim 22, wherein the first opening perimeter wall has a portion that curves inwardly and away from the heat exchange conduit.
 24. The heat exchanger of claim 23, wherein the second opening perimeter wall has a portion that curves inwardly and away from the heat exchange conduit.
 25. The heat exchanger of claim 22, wherein the first and second opening perimeter walls each include a first portion that curves inwardly and toward the heat exchange conduit, a second portion that curves inwardly and away from the heat exchange conduit, and a third portion that connects the first portion and the second portion, the first portion being closer to the respective outside surface of the first plate and second plate than the second and third portions.
 26. The heat exchanger of claim 25, wherein the third portions of the first and second opening perimeter walls contact the respective end portions of the heat exchange conduit.
 27. The heat exchanger of claim 25, further comprising an adhesive or sealant that is provided between (i) the first portion of the first opening perimeter wall, and (ii) the end portions of the heat exchange conduit.
 28. The heat exchanger of claim 27, wherein the adhesive or sealant is added by rolling it across the first outer surface of the first plate.
 29. A heat exchanger for indirect evaporative cooling of a gas stream, comprising: at least one elongate heat exchange conduit; a first plate with a first outside surface and a first inside surface, the first plate having at least one first opening that defines a first opening perimeter wall in the first plate; wherein: one end portion of the elongate heat exchange conduit is fitted into the first opening and in contact with the first opening perimeter wall, and the first opening perimeter wall curves inwardly from the first outside surface of the first plate in the direction of elongation of the heat exchange conduit, and the first opening perimeter wall includes a portion that curves inwardly and away from the heat exchange conduit.
 30. A method of making the heat exchanger according to claim 22 comprising: fitting the one end portion of the heat exchange conduit into the first opening so that one end of the heat exchange conduit is substantially flush with first outside surface of the first plate; fitting the another end portion of the heat exchange conduit into the second opening so that another end of the heat exchange conduit is substantially flush with the second outside surface of the second plate; and rolling an adhesive or sealant across at least one of the first outside surface and the second outside surface. 